1. Field of the Invention
This invention relates generally to fixed frequency switching regulators, and more particularly to such a regulator having improved dynamic response, a low count of external discrete elements, can be used at 100% duty cycle, does not require a minimum load, and does not require compensation components.
2. Description of Related Art
The following art defines the present state of this field:
Ghotbi, U.S. Pat. No. 5,233,509 describes an adjustable device with near unity power factor that is achieved while reducing harmonics in a switch-mode power supply by replacing the conventional diode bridge in the front end with an H-bridge driver. Two feedback loops are required, one feedback loop is used to regulate a DC bulk voltage by comparing the DC bulk voltage to a reference voltage and generating an error signal therefrom. A programmable offset is applied to the error signal for adjusting the power factor. A second feedback loop samples and scales the AC input voltage for use as a scaled replica of the input current The actual input current is then compared to the scaled replica current to produce a second error signal. The two error signals are multiplied to determine the desired input current wherein the desired input current follows the AC input voltage. The desired input current is pulse width modulated by a fixed frequency sawtooth waveform for providing switching signals for controlling the H-bridge driver. The H-bridge driver thus operates such that the input current closely follows the AC input voltage and near unity power factor is maintained. A DC-to-DC converter follows the H-Bridge driver using a single feedback loop for regulating desired DC output voltage.
Shelly, U.S. Pat. No. 4,302,803 describes an AC to DC power supply having a multi-channel, flyback inverter output section regulated via a fixed frequency, pulse width modulation controller. The controller directly regulates each channel by regulating an auxiliary channel, whereby the duty cycle of a switching transistor in the flyback inverter section is varied to maintain a constant DC voltage on the auxiliary channel and correspondingly on each output channel.
Harris, U.S. Pat. No. 5,489,831 a pulse width modulation controller particularly useful for a variable speed variable torque electric motor. Inputs to the controller are a desired RPM signal, a motor electrical signal which is a function of the RPM of the rotor of the motor and its angular position relative to the stator, and a torque feed back signal derived from the power phase winding circuits of the motor. The controller produces pulse width modulated power drive signals which are applied to power switches of the power phase winding circuits of the motor to allow electric current to flow through the power phase winding circuits when power phase enable signals produced by the controller sequentially enable the power switch of a power phase winding circuit to do so. The duty cycle of the power drive signals is a function of the difference between the desired and actual RPM of the motor and the torque of the motor. The frequency of the power drive signals is a fixed integral multiple of the frequency of the power phase enable signals over the fill operating range of RPMs of the motor.
Mammano et al., U.S. Pat. No. 5,414,342 a pulse width modulator voltage feedforward circuit which includes a gating circuit for providing a control signal to a charge circuit which charges an integration circuit with a current proportional to an input voltage. The integration circuit provides an output ramp signal waveform having a slope proportional to the input voltage value. The gating circuit also provide a control signal to a discharge circuit, to thus alternately charge and discharge the integration circuit The gating circuit insures that a minimum deadtime for a transformer reset will occur regardless of input voltage variations by preventing a fixed frequency signal provided by an oscillator from beginning a new ramp waveform signal period until the integration circuit is discharged to a minimum reference voltage level.
Green et al., U.S. Pat. No. 5,404,094 describes a method and system of converting a wide range of DC input voltages from a DC power source to a substantially lower regulated DC output voltage. A pulse width modulated (PWM) signal is generated from an applied DC input voltage to obtain a desired regulated DC output voltage. The PWM signal has a fixed frequency and a variable duty cycle. The PWM signal is filtered to provide a DC output voltage proportional to the amount of time the PWM signal spent in the ON or HIGH state. The duty cycle of the PWM signal is controlled based on an error signal generated by comparing an error voltage level that is proportional to the output voltage to a voltage reference. High voltage protection is provided to the controller during the period the PWM signal is in the OFF or LOW state.
The prior art teaches the use of a pulse-width modulated controller for several applications. However, the prior art does not teach that such a controller may be used for controlling a fixed frequency switching regulator. The present invention fulfills these needs and provides further related advantages as described in the following summary.